The present invention relates to the manufacture and packaging of semiconductor light emitting diodes (“LED”). An LED is a semiconductor device that emits light whenever current passes through it. In its simplest form, a light emitting diode includes a p-type portion and an n-type portion to define a p-n junction diode. When mounted on a lead frame and encased in an encapsulant (usually a polymer), the overall LED package is also referred to as a “lamp.”
Because of the high reliability, long life and generally low cost of LEDs, they have gained wide acceptance in a variety of lighting applications in many fields of application.
LED lamps are extremely tough. They typically do not include glass and avoid filaments entirely. As a result, LED lamps can take abuse far beyond that of the incandescent lamp and their high reliability can greatly reduce or eliminate many maintenance factors and costs.
LED lamps can be extremely efficient, e.g., emitting light equal to an incandescent lamp while consuming only 10 percent of the electricity. Many LEDs have life spans of 100,000 hours; i.e. equivalent to over 11 years of continuous use. Therefore, from a statistical standpoint, most LED's will never fail once they are initially tested (typically as part of the production process). LED lamps are excellent for use in unusual or difficult environments such as near explosive gases or liquids. Although individual light choices (solid state versus incandescent or fluorescent) still must be designed and tested for each particular use, as a general rule, LED lights are a safer choice in a wide variety of applications.
LED lamps are energy efficient and environmentally friendly. They minimize the use of electricity and batteries, and their relatively low current requirements means they can be solar powered more easily.
The nature, structure and operation of LEDs is generally well-understood. A conceptual discussion and understanding of the nature and operation of light emitting diodes and the physics and chemistry that support their operation, can be found for example in textbooks such as Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Ed. (1981) and Sze, MODERN SEMICONDUCTOR DEVICE PHYSICS (1998).
A number of commonly assigned patents and co-pending patent applications likewise discuss the theory and nature of light emitting diodes, including but not limited to U.S. Pat. Nos. 6,459,100; 6,373,077; 6,201,262; 6,187,606; 5,912,477; 5,416,342; and 5,838,706; and Published U.S. Applications Nos. 20020022290; 20020093020; and 20020123164. The contents of these are incorporated entirely herein by reference.
As all of these sources attest, the color emitted by a light emitting diode depends upon the nature of the semiconductor material from which it is formed. As particularly set forth in the commonly assigned patents and applications, light in the green, blue, violet, and ultraviolet portions of the electromagnetic spectrum has higher energy compared with red or yellow light. Such high energy light can typically only be generated using materials having a wide band gap, that is, a bandgap sufficient to create photons with the required energy. (“Bandgap” is an intrinsic quality of a semiconductor material that determines the energy released when a photon is generated in the material.) Silicon carbide, gallium nitride, and other Group III nitrides, as well as certain II-VI compounds such as ZnSe and ZnS are examples of wide-bandgap semiconductor materials capable of generating blue, green and/or UV light. As further set forth in the incorporated references, of these materials, gallium nitride and other Group III nitrides have begun to emerge as favorite materials for LED production.
For a number of packaging and use applications, a favored design for a light emitting diode is the “vertical” orientation. The term “vertical” is not used to describe the final position of the overall device, but instead to describe an orientation within the device in which the electrical contacts used to direct current through the device and its p-n junction are positioned on opposite faces (axially) from one another in the device. Thus, in its most basic form, a vertical device includes a conductive substrate, a metal contact on one face of the substrate, two or more epitaxial layers on the opposite face of the substrate to form the p-n light-emitting junction, and a top contact on the top epitaxial layer to provide a current path through the layers and their junction and the substrate to the substrate contact.
In the latest-generation LEDs produced by the assignee of the present invention, e.g., published U.S. Application No. 20020123164, the basic LED structure includes a silicon carbide substrate, an n-type gallium nitride epitaxial layer on the substrate, a p-type gallium nitride layer on the n-type layer, thereby forming a p-n junction and a metal stack on the p-type layer, which also forms the top contact to the device. It has been found that the emission of light from such devices can be enhanced by carefully selecting the transparency and geometry of the substrate to maximize the emission of light based upon its expected wavelength and the index of refraction of the silicon carbide substrate and potentially that of the packaging material. Accordingly, in the latest commercial embodiments, the light emitting diode is positioned on a lead frame with the epitaxial layers of the diode adjacent the lead frame with the silicon carbide substrate above them. This orientation is sometimes referred to as “flip chip” or “junction down” and will be discussed in more detail with respect to the drawings. The leadframe is the metal frame onto which a die is attached and bonded. Parts of the leadframe may become the external connections of the circuit.
Although the “flip chip” design is advantageous, it may result in a very small tolerance or space between and among the lead frame, the die attachment metal, the metal contact layers of the device, and the terminal edges of the epitaxial layers. Because the epitaxial layers include and define the p-n junction, the tolerances between the metal and the junction can be as small as 1-5 microns. Accordingly when the LED is mounted in a substrate-up, junction-down orientation on the lead frame, and with a metal (or other functionally conductive material) being used to provide an electrical contact between the lead frame and the ohmic contact to the p-type portion of the diode, the metal used to attach the LED to the lead frame can inadvertently make contact with the n-type layer and form a parasitic (i.e. unwanted) metal-semiconductor connection known to those skilled in the art as a Schottky contact.
Additionally, the passivation layer (typically silicon nitride) that is often added to protect the diode can crack following thermal or mechanical stress and thus provide additional possibilities for the development of undesired contacts to the epitaxial layers of the device.
By way of comparison and explanation, the problem described is essentially non-existent when diodes are positioned on the lead frame with the substrate rather than the epitaxial layers adjacent the lead frame. In such cases, the direct electrical contact between the die attachment metal and the (typically) n-type silicon carbide substrate is of course desired in order to provide current flow through the substrate and the junction.
In a typical LED manufacturing process, epitaxial layers of one or more semiconducting materials are grown on a semiconductor substrate wafer. Such wafers are typically between 2 inches and 4 inches in diameter, depending upon the semiconductor materials being used. Because individual LED die are typically quite small (e.g., 300×300 microns), a large quantity of LED die may be formed on a substrate wafer and its epitaxial layers in a geometric grid pattern. In order to successfully produce individual devices, the LED die in the grid must be separated from one another, both physically and electrically. Once the LEDs have been formed on a wafer, they are then separated into individual die, or groups of die, using well understood separation techniques such as sawing, scribe-and-break or the like.
The process of die separation may be harmful to exposed p-n junction regions. Therefore, prior to separation, it is known to isolate individual die while they remain on the wafer. The most typical method of isolation, which also serves to clearly define the devices and the location for their ohmic contacts, is to carry out one or more photolithography steps and etching the epitaxial layers to define a junction-containing mesa for each device or device precursor.
Although photolithography is a useful technique in semiconductor design and manufacturing, it requires specific equipment and materials and adds process steps. For example, a typical photolithography process can include the steps of adding a layer of photoresist (typically a polymer resin sensitive to light) to a semiconductor structure, positioning a mask over the photoresist, exposing the photoresist to a frequency of light to which it responds (by undergoing a chemical change; usually its solubility in a particular solvent), etching the photoresist to remove the exposed or unexposed pattern (depending upon the resist selected), and then carrying out the next desired step on the remaining pattern. In particular, when the purpose of the patterning step is to define an etch pattern in a GaN-based layer, GaN's chemical, physical, and thermal stability (which are favorable characteristics in finished devices) can cause additional difficulties if the etchant removes the resist before fully removing the desired pattern of material.
Accordingly, forming mesa-type LEDs that include a top contact metal layer will typically require at least two full sets of these steps; one set for patterning and etching the mesa and another set for patterning and depositing the metal contact layer.
Therefore, improvements in isolating devices from one another can provide corresponding improvements in the structure and performance of LEDs and LED layers.